The present invention relates to a circuit which may be monolithically integrated for measuring longitudinal and transverse currents in a two-wire transmission line and may be used, in particular, in electronic telephone interface circuits between a subscriber's telephone line and exchange control components.
As shown in the technical literature dealing with telephone interface circuits of this type, known also by the abbreviation SLIC (Subscriber Line Interface Circuit), the transfer of functions previously carried out by devices incorporated in individual subscribers' sets to electronic circuits at the exchange, has made accurate measurement of the line currents at the time of connection of the subscriber's line indispensable.
The problems entailed in accurately detecting the directions and intensities of longitudinal and transverse line currents so that appropriate regulation circuit means may be supplied with this information via measurement signals, have arisen, in particular, from the need to synthesize impedances, using electronic circuit means exclusively, so as to enable automatic line tuning in all operating and network conditions and from the need to be able to detect, with absolute certainty, the working signals transmitted in the line such as, for example, the lifting or replacement of a subscriber's telephone receiver, at the exchange.
As known to persons skilled in the art, an electronic interface circuit between a subscriber's telephone line and exchange control components in general comprises a circuit structure of the bridge type formed by two output amplifier components between which the subscriber's telephone line, and all the apparatus connected thereto, is inserted as a load.
These amplifier components drive the line in phase opposition when signals are present.
The "transverse" line current I.sub.L is the sum of the direct current supplying the line and the signal current which is generally of an alternating type. This transverse current I.sub.L is of identical intensity in the two wires of the line, but has opposite directions of flow.
However, if electrical lines having a 50 Hz alternating current or frequencies of an industrial type, or other telephone lines in which high intensity signals, such as ringing signals, are being transmitted, are in the vicinity of a two-wire telephone line, or more generally a two-wire transmission line, they may induce "longitudinal" or "common mode" currents I.sub.CM in both wires of this line, these currents having identical intensities and directions of flow in both wires of the line.
These common mode currents I.sub.CM which may, in the cases discussed above, be of an alternating type, do not, in general, have a waveshape which is fixed over time, therefor when they are superimposed on the transverse line current I.sub.L, they alter its value in an unpredictable way.
If the overall resultant currents in the wire of the line with the higher potential and the wire of the line with the lower potential are designated conventionally by I.sub.A and I.sub.B respectively, the following may be expressed: EQU I.sub.A =I.sub.L +I.sub.CM EQU I.sub.B =I.sub.L -I.sub.CM
in which the actual directions of each current obviously have to be borne in mind in accordance with known electrotechnical conventions.
This shows that it is sufficient, in theory, to add and subtract the total currents I.sub.A and I.sub.B for an immediate "measurement" of the transverse line current I.sub.L and the longitudinal common mode currents I.sub.CM respectively. This gives: EQU .vertline.I.sub.A +I.sub.B .vertline.=2.vertline.I.sub.L .vertline. EQU .vertline.I.sub.A -I.sub.B .vertline.=2.vertline.I.sub.CM .vertline.
It is particularly necessary to obtain a measurement of the common mode currents I.sub.CM in cases in which these currents are intentionally induced in the line so that specific functions may be carried out, as takes place, for example, in private exchanges to enable the transfer of incoming calls from one subscriber's set directly to another by means of the appropriate key on the set itself.
In reality, a circuit capable of carrying out these simple operations in all longitudinal and transverse current conditions is comparatively complex with the result that it is expensive, if monolithically integrated, both from the point of view of integration area occupation and design problems.
In the adding and subtracting operations on the overall currents in the line are to give representative measurement results under all operating conditions, it is necessary, in the first instance, to take into account the possibility of inversions of the line polarity, since it is this polarity which determines the direction of flow of the transverse current I.sub.L in the line.
In addition, the possibility, particularly in very long transmission lines, in which the transverse line current I.sub.L is obviously reduced, while the probability of longitudinally induced currents is higher, of the intensity of the longitudinal common mode currents I.sub.CM being greater than the intensity of the transverse line current I.sub.L, should not be neglected.
In this case, the overall currents in the line I.sub.A and I.sub.B have the same direction which is not determined by the line polarity, but is variable over time in a uniform manner with the direction of the common mode currents induced.
Since the active electronic components conduct essentially in a monodirectional manner during normal polarization and operating conditions, a circuit which may be monolithically integrated for measuring longitudinal and transverse line currents must be designed in such a way that it has an overall configuration which is compatible with input currents having any direction.